This study compared the effects of standard monophasic versus biphasic direct current shocks for cardioversion of atrial fibrillation (AF) on the release of cardiac troponin I (cTnI) and myoglobin (Myo). We randomized 48 patients with persistent AF (mean age = 61.4 +/- 10.7 years, 33 men) to monophasic (45.2%) or biphasic (54.8%) cardioversion. Plasma concentrations of cTn1 and Myo were measured before, and 6 and 24 hours after the procedure. Cardioversion was significantly more effective (88% vs 100%, P < 0.04) and required less energy (348.1 +/- 254.1 vs 187.6 +/- 105.3 J; P < 0.001) in the biphasic than the monophasic group. A significant increase in mean plasma cTnI concentration over 24 hours (0.23 +/- 0.18 vs 0.41 +/- 0.37 ng/mL, P < 0.04), and mean Myo concentration were recorded in the monophasic group over the first 6 hours following the procedure (38.2 +/- 14.2 vs 221.9 +/- 51.3 ng/mL, P < 0.001), whereas no significant increase was observed in the biphasic group. Increases in cTnI and Myo in the monophasic group correlated closely with the cumulative energy delivered (Spearman correlation coefficient r = 0.58, P = 0.004 for Myo and r = 0.67, P < 0.001 for cTnI). In addition, there was a positive correlation between cumulative cardioversion energy load and increase in Myo and cTnI indexed with left ventricular mass (r = 0.45, P < 0.02 for Myo and r = 0.47, P = 0.01 for cTnI). It is concluded that in cardioversion of AF, biphasic are more effective than monophasic and may cause less myocardial injury.